This invention relates generally to chemical vapor deposition of material on semiconductor wafers, and more particularly to an improved susceptor for supporting wafers in a reaction chamber of a barrel reactor during a chemical vapor deposition process.
Chemical vapor deposition is a process for growing a thin layer of material on a semiconductor wafer so its lattice structure is identical to that of the wafer. Using this process, a layer having different conductivity may be applied to a semiconductor wafer to achieve desired electrical properties. Generally, chemical vapor deposition is accomplished by introducing a reactant gas comprising the deposition material (e.g., silicon) and a carrier gas into a reaction chamber of a barrel reactor containing wafers.
The wafers are held in the reaction chamber on a susceptor so one face of the wafer is exposed to the reactant gas. The susceptor is suspended inside the reaction chamber and slowly rotates to evenly distribute the reactant gas over the wafers. Although other shapes are used, susceptors are generally prismatic and have several (e.g., five) planar faces. A face of a conventional susceptor is shown in FIG. 1 and is designated by the reference numeral 10. Circular recesses 12 are formed in the face 10 for holding wafers W in a generally vertical orientation (i.e., so they face generally laterally outward). Conventional susceptors are slightly tapered so their tops are smaller than their bottoms. This tapered configuration allows the wafers to lean inward into the recesses so gravity holds the wafers generally upright against the susceptor. The recesses have a slightly larger diameter than the diameter of the wafers to provide clearance for grasping the wafers when removing them from the recesses. For instance, a susceptor for processing 150 mm diameter wafers may have recesses which are approximately 160 mm in diameter. Two rectangular depressions 16 near the top of each recess 12 provide addition clearance for grasping the wafers. In order to reduce the height of the susceptors, the recesses overlap as shown in FIG. 1.
Reactant gas not only deposits on the wafers but also on many interior features of the reaction chamber, and it preferentially deposits on nucleation sites such as particles and sharp edges in the chamber. Due to the overlapped configuration of the recesses, sharp corners 14 are formed on the susceptor at the intersections of the recesses 12 as shown in FIG. 1. Reactant gas tends to deposit on these sharp corners 14. Because the wafers W rest against these corners 14, the deposits bridge the gap between the wafers and the corners of the susceptor. When the wafers are removed from the susceptor after the chemical vapor deposition process is complete, these bridges hold the wafers against the susceptor, sometimes cracking or chipping the wafers as they are removed from the susceptor. These cracks or chips frequently cause the wafers to fail during later processing. In addition, the bridges occasionally cause the corners on the susceptor to break. The broken susceptor corners increase the propensity for gas to deposit at the corners. Thus, broken corners necessitate early replacement of the susceptors which causes more frequent production down time.